Theres a great vid on youtube from GoCardless which helps explain how many days it takes.
Basically;
If you process ur payments on a friday.
Customers are notifies on monday and so are their banks. 3 working days to request money from a bank. On this example, the cuatomers accounts are debited on wednesday. However GoCardless keeps the funds for near on 2 days, for various reasons. Then you get paid by GoCardless on Friday. So from processing to receiving payment, should be around 7 days.